You may have noticed something very strange about birthdays in movies and on TV. Namely, that the lucky recipient of the presents rarely has Happy Birthday sung to them – instead they might get For He’s a Jolly Good Fellow, a song only heard in movies and on TV. That’s because Happy Birthday – often claimed to be the world’s most popular song – has been under copyright to the publisher Warner Chappell, which has zealously enforced its right to royalties and earns an estimated $2m a year from Happy Birthday.

Now, however, it seems as though Warner Chappell might have made a mistake. Two years ago, film-makers working on a documentary about the song filed a lawsuit claiming the song should not be under copyright, after being told they would have to pay $1,500 to use it in their film. Director Jennifer Nelson has been seeking the return of that money and is also representing a class of plaintiffs who have also been charged for using the song.

Now attorneys for Nelson’s Good Morning to You Productions have found a songbook from 1927 containing Happy Birthday, with no copyright notice – predating Warner Chappell’s copyright by eight years. The songbook was in documents handed over by the publisher this month, which were “mistakenly” not produced during the discovery period in the case, which ended more than year ago.

In a motion filed in district court in California calling for summary judgment in favour of the film-makers, their lawyers say of this iteration of the song: “It and earlier versions of the song that Plaintiffs subsequently located through their own investigative efforts conclusively prove that any copyright that may have existed for the song itself (ie the setting of the Happy Birthday lyrics to the melody of Good Morning) expired decades ago. This newly discovered evidence is fully consistent with Plaintiffs’ arguments that (i) Patty Hill gave the lyrics to the public when she wrote them as a version of the song she wrote with her sister Mildred Hill and (ii) that the 1935 copyrights covered only specific piano arrangements of the song. More importantly, it trumps all of Defendants’ arguments.”

The motion argues that when music and lyrics are published with the authors’ permission, but without a copyright notice – as happened with the 1927 edition of the songbook, and earlier ones that the film-makers found, dating back to 1922 – then they fall “irrevocably” into the public domain. All the 1935 copyright should cover, they say, is any arrangement or alteration made by the publisher, not the song itself.

Happy Birthday was written by sisters Parry and Mildred Hill in 1893, adding new lyrics to a song they had already written – though that claim has been disputed – to a song called Good Morning to All. The 1935 copyright claim came from the Summy Company, and ownership of that copyright passed to Warner Chappell in 1988. The publisher says copyright will expire in the US in 2030, though it will enter the public domain in the EU by 31 December 2016.

This article was amended on 24 October 2015 to correct the year Happy Birthday was written in from 1883 to 1893.